This invention relates to a new and improved method of insulating electromagnetic coils, and it relates more particularly to a method of forming virtually void-free ground insulation around helical multi-turn coils such as used on the exciting and commutating field poles of a large direct current (d-c) motor.
In a large d-c motor, such as those used in the electrical propulsion systems of rapid transit cars, a plurality of field coils are respectively mounted on salient pole pieces that extend radially inwardly from the cylindrical magnet frame or stator of the motor. Each of these coils comprises a long copper conductor of relatively large cross-sectional area (e.g. one-quarter square inch) that has been wound or bent to form a plurality of juxtaposed turns (e.g. 17 turns). Typically the copper conductor has a rectangular cross-section and is wound edgewise, i.e. the wider side of the conductor is perpendicular to the centerline of the coil. The resulting multi-turn coil has an oblong helical shape, and its open center or "window" is appropriately dimensioned to fit around the associated pole piece which has a generally rectangular cross-section. The terminal sections of the copper conductor at opposite ends of the coil serve as relatively flexible leads that are connected via interconnection straps and external cables to a source of unidirectional electric current, and when energized or excited by such current the coil produces a desired magnetic field inside the motor.
Between adjacent turns of the field coil suitable insulating material is disposed so as to prevent turn-to-turn electrical short circuits, and the exposed surfaces of the whole stack of helical turns are encapsulated in such material to insulate the coil from the grounded pole piece and frame of the motor. The insulating system preferably is characterized by high dielectric strength, good heat transfer properties, both physical and chemical stability at elevated temperatures, and a high resistance to moisture and dirt. The heat transfer properties are particularly significant in traction motors where the goal is to obtain more output torque per unit of weight by increasing the current density (and hence the heat generated) in the coils.
Any air pockets or "voids" inside the insulating system will impede heat transfer and are therefore undesirable. After applying ground insulation to a field coil, assembling the coil on its pole piece, and bolting the pole piece inside the magnet frame of the motor, small voids have been discovered in the ground insulation. Such voids are caused by a tendency of the insulation to be slack or "baggy" on the outside of the coil. The bagginess may be the result of somewhat stiff ground insulation that does not closely conform to the contour of the stack of helical turns, and it is aggravated by the reduction in coil height that takes place when the pole piece is fastened to the frame and the turns of the coil are tightly clamped together between the frame and the overhanging head of the pole piece. It is difficult to completely fill the voids with varnish or resin during the vacuum-pressure impregnation (VPI) process that is conventionally used to improve the heat transfer and moisture-resistance properties of motor insulating systems.
The VPI treatment and its advantages are well known to persons skilled in the art of insulating coils and windings of electrical machines. Typically this process is executed after the field coil-pole piece subassemblies have been installed in the magnet frame and the frame has been pre-baked to dry out the insulating system. The basic steps of a VPI process comprise: placing the pre-baked frame in a sealed empty tank, drawing a vacuum to expel air from the interstices of the coil turns and insulating system, pumping enough flowable, solventless varnish into the tank to cover the coils, removing the vacuum, pressurizing the tank to force the varnish to penetrate the insulating system and fill the voids therein and any air spaces around and between the turns of the coil, draining the varnish from the tank which is then vented to atmospheric pressure, and baking the frame in an oven until the varnish cures. The amount of varnish that the insulating system retains depends on several factors, including the number and size of the voids. If the voids initially have too great a magnitude, as can result from the above-mentioned bagging of the ground insulation, they will not be totally filled with varnish during a commercially practical VPI process.
U.S. Pat. No. 3,600,801--Larsen et al discloses a motor coil of the random-wound type wherein multiple turns of magnet wire are rather loosely bundled in a coil to be encapsulated in ground insulation. In order to compress the bundle of wires and force varnish into the voids between adjacent turns, the bundle is encircled by a spirally wrapped heat-shrinkable polymeric tape which will shrink when subsequently heated during the VPI process. The tape is applied by "skip-taping" so as to leave space between its adjacent turns for varnish to permeate the coil. The ground insulation is applied later. In any event, the spirally wrapped heat-shrinkable tape would not be very effective to prevent the above-mentioned bagging of ground insulation on the outside of the coil because there is relatively little shrinkage in that region.
U.S. Pat. No. 3,297,970--Jones discloses a transformer coil covered with a sheet of glass cloth impregnated with a semi-cured (B stage) polyester resin that requires heat and pressure for final curing. Before being so covered, the coil was given a VPI treatment to impregnate the interstices between its turns with varnish, and afterwards the coil is encapsulated in a heat-shrinkable wrapping that applies pressure on the underlying cover when subsequently heated to the curing temperature of the resin.